The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which is provided with an auxiliary light source device having a light guide with solid-state light-emitting elements such as light-emitting diodes disposed at a side edge of the light guide as a light source.
In small-sized information terminals such as mobile telephones or mobile information terminals, it is general practice to adopt, as their display devices, liquid crystal display devices which are energy efficient, small-sized and light weight. The liquid crystal display devices are classified into two types: one type in which external light is utilized as illuminating means for visualizing an electronic latent image formed on a liquid crystal display panel; and the other type in which an auxiliary illuminating device is disposed on the back or front surface of a liquid crystal display panel. The auxiliary illuminating device disposed on the back surface of the liquid crystal display panel is called a backlight, while the auxiliary illuminating device disposed on the front surface of the liquid crystal display panel is called a frontlight.
As a light source for this kind of auxiliary illuminating device of such a mobile terminal, there is a light source which has a light guide with a cold cathode fluorescent tube disposed at a side edge of the light guide, as in a notebook personal computer having a comparatively large display screen size. However, many mobile telephones and small-sized mobile information terminals (such as PDAs) use solid-state light-emitting elements such as light-emitting diodes (LEDs) with low power consumption.
FIG. 11 is a diagrammatic explanatory perspective view of the entire construction of a related-art auxiliary illuminating device in a small-sized information terminal. This auxiliary illuminating device is disposed on the back surface of a liquid crystal display panel. The liquid crystal display panel, a printed circuit board and the like are not shown. Two prism sheets 2 are disposed directly below the liquid crystal display panel, and a diffusing sheet 3, a light guide 4 and a reflecting sheet 10 are disposed in that order below the two prism sheets 2. Reference numeral 8 denotes a light-emitting element which emits white light, and in general, a light-emitting diode (LED) is used. In the following description, this kind of light-emitting element is referred to as an LED.
FIGS. 12A and 12B are explanatory views of a structural example in which LEDs constituting the light source of the related-art auxiliary illuminating device are disposed in a small-sized information terminal. FIG. 12A is a plan view, and FIG. 12B is a cross-sectional view taken along line A—A of FIG. 12A. In FIGS. 12A and 12B, reference numeral 4 denotes a light guide, reference numeral 4A denotes a side edge (hereinafter referred to also as an entrance surface). Reference numeral 6 denotes a box-like case (molded case), and reference numeral 8 denotes an LED. In this structural example, one or two or more LEDs 8 are disposed on the inside wall of the molded case 6. In this example, three LEDs 8 are arranged.
However, in this construction, since the light guide 4 and the LEDs 8 are incorporated in the molded case 6 as separate components, it is difficult to mount the LEDs 8 accurately on the inside wall of the molded case 6, and a variation easily occurs in a distance d between the light-emitting portions of the respective LEDs 8 (that lie on the surfaces of the respective LEDs 8 opposed to the entrance surface 4A of the light guide 4) and the entrance surface 4A of the light guide 4. In addition, the light-emitting parts and the entrance surface 4A of the light guide 4 are difficult to oppose precisely directly. For this reason, the distribution of light which is emitted from the LEDs 8 and enters the light guide 4 becomes nonuniform therein, so that irregularity may occur in the display luminance of the liquid crystal display panel. This has been one problem to be solved in the related art.
Moreover, because of the structure in which the LEDs 8 are mounted in the molded case 6, wiring means is needed for connecting power supply lines for the LEDs 8 to power source terminals (or power source electrodes) of driver circuits mounted on a printed circuit board (not shown), so that the assembling process becomes complicated. This has been another problem to be solved in the related art. Incidentally, this kind of related art is disclosed in, for example, Japanese Patent Laid-Open No. 133426/1999.
To solve the above-described problem, various proposals which will be described below have been made. FIG. 13 is a diagrammatic explanatory cross-sectional view of an example of the essential construction of a proposed example of an auxiliary illuminating device in a small-sized information terminal. FIG. 13 mainly shows the light guide 4 of the auxiliary illuminating device and the LEDs 8 which are light-emitting elements. In this construction, the LEDs 8 to be disposed in proximity to or in intimate contact with one end (the side edge 4A) of the light guide 4 suitably made of a transparent resin are mounted on a printed circuit board 5, and this printed circuit board 5 is disposed on the back surface of the light guide 4. Incidentally, as the printed circuit board 5, a non-flexible type made of a hard plate or a type made of a flexible sheet (a so-called flexible printed circuit board) is available, and in the following description, both types will be collectively referred to as “printed circuit board”.
Since the LEDs 8 are ordinarily mounted on the printed circuit board 5 by an automatic mounting apparatus, their mounting positions can be controlled accurately. In addition, the light guide 4 is secured to the back surface of a liquid crystal display panel (not shown) by an adhesive agent or a double-sided adhesive tape, and the printed circuit board 5 is constructed to be connected to the liquid crystal display panel. Accordingly, light-emitting portions 8A of the respective LEDs 8 can be disposed to be accurately directly opposed to the side edge 4A which is the entrance surface of the light guide 4. Incidentally, although not shown, a reflection-processed film or a reflecting sheet are disposed on the back surface of the light guide 4.
FIG. 14 is a diagrammatic explanatory view of an example of the essential construction of a liquid crystal display device, aiding in explaining the problem of the construction shown in FIG. 13. FIG. 14 is an explanatory cross-sectional view particularly showing the arrangement relationship between the printed circuit board 5 on which the LEDs 8 a mounted as the light-emitting elements of the auxiliary illuminating device, and the light guide 4 as well as a liquid crystal display panel 1. The LEDs 8 are disposed in proximity to or in intimate contact with the side edge 4A of the light guide 4 disposed on the back surface of the liquid crystal display panel 1. Although the printed circuit board 5 is herein described as a hard printed circuit board, the same description applies to a flexible printed circuit board.
The LEDs 8 are mounted on the printed circuit board 5 with the light-emitting portions 8A facing the side edge 4A of the light guide 4. Light emitted from the light-emitting portions 8A of the LEDs 8 is directed in a direction approximately parallel to the plane of the printed circuit board 5. The respective LEDs 8 have electrodes 8B at their bottoms, and these electrodes 8B are connected to the wiring of the printed circuit board 5, whereby the LEDs 8 are secured and electrically connected to the printed circuit board 5. Other electronic components 9 are mounted on the surface of the printed circuit board 5 opposite to the surface on which the LEDs 8 are mounted. In other words, the LEDs 8 and those other electronic components 9 are mounted on the opposite sides of the printed circuit board 5, and this mounting method is called dual-sided mounting.
FIG. 15 is a diagrammatic explanatory view of an example of the essential construction of another proposed example of the auxiliary illuminating device in the small-sized information terminal. FIG. 15 is an explanatory cross-sectional view particularly showing the arrangement relationship between the printed circuit board 5 on which the LEDs 8 are mounted and the light guide 4 as well as the liquid crystal display panel 1. The LEDs 8 are mounted on the same surface of the printed circuit board 5 as the other electronic components 9 and at a portion near one end of the printed circuit board 5. The portion where the LEDs 8 are mounted is folded in the direction of the liquid crystal display panel 1, and in a manner similar to that shown in FIG. 14, the respective light-emitting portions 8A of the LEDs 8 are disposed in proximity to or in intimate contact with the side edge 4A of the light guide 4 disposed on the back surface of the liquid crystal display panel 1, in such a manner that the light-emitting portions 8A are opposed to the side edge 4A.
The LEDs 8 are mounted on the printed circuit board 5 with the light-emitting portions 8A facing the side edge (entrance surface) 4A of the light guide 4. Light emitted from the light-emitting portions 8A is directed in a direction approximately parallel to the plane of the printed circuit board 5 and enters the entrance surface 4A of the light guide 4. The respective LEDs 8 have the electrodes 8B at their bottoms, and these electrodes 8B are connected to the wiring of the printed circuit board 5, whereby the LEDs 8 are secured and electrically connected to the printed circuit board 5. The other electronic components 9 are mounted on the same surface as the surface where the LEDs 8 are mounted. In this example, the LEDs 8 and the other electronic components 9 are mounted on the same surface of the printed circuit board 5, and this mounting method is called single-sided mounting.
Incidentally, a related art associated with the mounting of the LEDs 8 on the printed circuit board 5 is disposed in Japanese Patent Laid-Open No. 77724/2000. This laid-open document does not have any disclosure as to liquid crystal display devices. The mounting art disclosed in this laid-open document relates to the mounting of a single light-emitting element or of a light-emitting element and a light-receiving element, and aims at reducing the total thickness of a printed circuit board. However, the light-emitting direction of the light-emitting element is a direction perpendicular to the plane of the printed circuit board, and is not intended to solve problems to be solved by the invention, which occur when emitted light is made to enter a light guide in a direction parallel to a printed circuit board.
The example of mounting the LEDs in the auxiliary illuminating device according to each of the above-described proposals has the following problems. For instance, in the case of the dual-sided mounting in which the LEDs 8 are mounted on the surface of the printed circuit board 5 opposite to the surface where the other electronic components 9 are mounted, as shown in FIG. 14, mounting work for the printed circuit board 5 is complicated, and the process of performing mounting on two surfaces is needed in the automatic mounting apparatus. In the case where the single-sided mounting type of printed circuit board 5 described above with reference to FIG. 15 is used, the portion where the LEDs 8 are mounted needs to be folded so that the light-emitting portions 8A are opposed to the side edge 4A of the light guide 4. Accordingly, a folding process is needed, so that work efficiency lowers and cracks or the like may occur in a folded portion, incurring malfunction. In addition, the thickness of the auxiliary illuminating device increases, with the result that this increase of the thickness hinders a further reduction in the entire thickness of a liquid crystal display device in which such an auxiliary illuminating device is incorporated, and hence a further reduction in the thickness of a terminal using such a liquid crystal display device.
Moreover, in each of the above-described mounting methods, since the LEDs 8 are mounted on the mounting surface of the printed circuit board 5, the distance between the light-emitting portions 8A of the respective LEDs 8 and the surface of the printed circuit board 5 can only be reduced within a limited range beyond which no further reduction in thickness is possible. Since the respective light-emitting portions 8A of the LEDs 8 lie inside the molded surfaces of the LEDs 8, the thickness of the light guide 4 can only be made smaller than the size of the light-emitting portion 8A of each of the LEDs 8 directly opposed to the light guide 4, within a limited range in terms of the efficiency of utilization of illuminating light. These limitations are also a cause which hinders a further reduction in the thickness of the auxiliary illuminating device as well as a further reduction in the thickness of the entire terminal in which the auxiliary illuminating device is incorporated.
The invention solves the problems of the above-described related arts and provides a liquid crystal display device in which light-emitting elements such as LEDs which constitute an auxiliary illuminating device can be mounted on a printed circuit board together with other electronic components by single-sided mounting and the light-emitting elements can be opposed to the entrance surface of a light guide without the need to fold the printed circuit board, whereby further reductions in the number of manufacturing steps and in the thickness of the liquid crystal display device can be promoted.